Studies of Attention-Deficit/Hyperactivity Disorder (ADHD) within birth-records-based Missouri population samples suggest that a severe combined subtype of ADHD defined by population-based criteria is a distinct form of ADHD deserving of further characterization. Specifically, this population-defined form of ADHD appears to be a genetically distinct disorder, with a characteristic developmental profile that includes elevated levels of autistic symptoms and motor problems. The current project aims to replicate and expand upon the above findings using available datasets plus additional assessments that will aid in characterizing the neurodevelopmental phenotype of children with severe combined type ADHD. Also, the project will use family history data to determine whether the combination of severe combined type ADHD plus motor and/or social deficits is an indicator of family genetic risk for psychiatric illnesses other than ADHD itself. Major hypotheses of this project include: 1) Severe combined type ADHD is associated with high rates of autistic traits and motor problems as measured by standard assessment tools, 2) Within individuals with severe combined type ADHD, the degree of motor impairment is associated with the degree of autistic traits. 3) Specific patterns of motor problems, autistic traits and ADHD symptoms will cluster within individuals and within families. 4) Individuals with severe combined ADHD plus motor and/or social deficits will have a stronger family history of psychiatric illness than individuals with severe combined ADHD alone. Refinement of naturally-occurring ADHD phenotypes involving autistic traits and/or motor problems may facilitate future genetic studies, longitudinal outcome studies, and clinical treatment trials. This project is relevant to public health because improved diagnostic classification of childhood and adolescent disorders such as ADHD and autism spectrum disorders may eventually lead to better screening, prevention, and treatment of such disorders within the population.